Metabolic engineering of microorganisms is an effective means to produce commercially a number of chemicals useful for a variety of applications, including production of polymer monomers and food additives (see, e.g., Lee, S. Y., et al. Macromol. Biosci. 4:157-164 (2004)).
As an example, fumaric acid is an organic acid widely found in nature. In humans and other mammals, fumaric acid is a key intermediate in the tricarboxylic acid cycle for organic acid biosynthesis (also known as the Krebs cycle or the citric acid cycle). Fumaric acid is also an essential ingredient in plant life. Fumaric acid is the strongest organic food acid in titratable acidity and in sourness. In one example, commercial fumaric acid is made from N-butane that is oxidized to maleic acid that is then isomerized to fumaric acid. Production of fumaric acid by bioprocess methods has potential to avoid synthetic production processes that often are more costly than bioprocess methods.
As an additional example, lactic acid (lactate) is used in the food industry as an additive for preservation, flavor, and acidity. It is also used for the manufacture of poly-lactic acid, a biodegradable plastic, and ethyl lactate, an environmentally friendly nonchlorinated solvent. Worldwide, in excess of 100,000 tons of lactic acid is produced annually, with predictions of an increasing demand. The growth in demand is attributable to the poly-lactic acid and ethyl acetate products.
In a number of microorganisms, lactic acid is normally produced from pyruvic acid (pyruvate). The reaction also occurs in the cells of higher organisms when oxygen is limited. Glycolysis is the sequence of reactions that converts glucose into pyruvic acid (pyruvate). Glycolysis can be carried out anaerobically. Pyruvic acid has a number of fates depending on where the chemical reaction takes place and whether the reaction takes place in the presence or absence of oxygen.
As shown in FIG. 1, under aerobic conditions, pyruvic acid can be converted to acetyl-CoA by pyruvate dehydrogenase. Under anaerobic conditions, pyruvic acid can be converted to ethanol (alcoholic fermentation) or lactic acid (e.g., in contracting muscle). The conversion of pyruvic acid to lactic acid is catalyzed by lactate dehydrogenase (LDH). The efficiency of lactic acid fermentation can be quantified as the percent yield of lactate from glucose or as a decrease in the levels of co-products (e.g., glycerol, ethanol, and fumarate) found in the fermentation broth.
Lactic acid is often manufactured using Lactobacilli, which typically has specialized growth requirements and is unable to produce significant amounts of lactic acid below pH 4. (Skory, C. D. J. Ind. Microbiol. Biotechnol. 30:22-27 (2003)). Alternatively, maintenance of neutral pH results in decreased product solubility in the form of salts and requires further processing to regenerate the acid from the resulting lactate salt.
Saccharomyces cerevisiae is a hearty, acid-tolerant microorganism that is amenable to industrial processes. In these microorganisms, however, the major product of pyruvate metabolism is ethanol, by way of pyruvate decarboxylase. Skory reported the production of lactic acid in a yeast, S. cerevisiae, expressing an ldh gene derived from Rhizopus oryzae. (J. Ind. Microbiol. Biotechnol. 30:22-27, (2003)). Skory demonstrated an increase in lactic acid production in the recombinant yeast. Nevertheless, despite the increase in lactic acid production, the majority of carbon was diverted into ethanol. In the same report, when lactic acid production was studied in a S. cerevisiae mutant strain deficient in ethanol production, diminished ethanol production was observed, but the efficiency of lactic acid production also decreased.
Anderson et al. demonstrated that ldh activity had little or no effect on the flux of carbon to lactic acid in Lactococcus lactis. Eur. J. Biochem., 268:6379-6389 (2001). Despite increasing the expression and activity of ldh to beyond that found in wild-type L. lactis, researchers observed no change in the flux of carbon to lactic acid.
Lactic acid can be synthesized chemically, but such synthesis results in a mixture of D and L isomers. The products of microbiological fermentation depend on the organism used and also may include a mixture of the two isomers or individual isomers in a stereospecific form. The desired stereospecificity of the product depends on the intended use; however, L-(+)-lactic acid is the form desired for most applications (Skory, C. D. Appl. Environ. Microbiol. 66:2343-2348 (2000)).
U.S. Pat. No. 6,528,636 describes R. oryzae (ATCC 9363) as a lactic acid producer found in the Rhizopus genus. Rhizopus is a filamentous fungus that is commercially versatile and used in the production of fermented foods, industrial enzymes such as glucoamylase and lipase, corticosteroids, chemicals such as glycerol and ethanol, as well as organic acids such as lactic acid and fumaric acid.
Production levels of different metabolites vary tremendously among the Rhizopus species, with some species producing predominantly lactic acid and others producing primarily fumaric acid. An ideal lactic acid-producing Rhizopus strain would produce little or none of these metabolites, since their production depletes sugars that could be used for conversion to lactic acid.
Ethanol is believed to be produced by most Rhizopus species primarily in low oxygen conditions. While Rhizopus is not typically considered an organism that grows under anaerobic conditions, it does possess ethanol fermentative enzymes that allow the fungus to grow for short periods in the absence of oxygen.
U.S. Pat. No. 4,877,731 discusses that fumaric acid production has been well studied in Rhizopus and that the fumarase gene also has been isolated. Synthesis of fumarate is believed to occur primarily through the conversion of pyruvate to oxaloacetate by pyruvate carboxylase. Conditions leading to increased fumaric acid usually are associated with aerobic growth in high glucose levels and low available nitrogen. Accumulation of fumarate often is a problem with lactic acid production, because its low solubility can lead to detrimental precipitations that compromise fermentative efficiency.
Glycerol is also a by-product that often is produced by Rhizopus grown in high glucose-containing medium. Glycerol is thought to accumulate in Rhizopus in a manner similar to that found in Saccharomyces (U.S. Pat. No. 6,268,189).
Oxaloacetate is also produced by Rhizopus. Pyruvate carboxylase [EC 6.4.1.1] is a member of the family of biotin-dependent carboxylases which catalyzes the carboxylation of pyruvate to form oxaloacetate with concomitant ATP cleavage. The resulting oxaloacetate can be used for the synthesis of glucose, fat, and some amino acids or other derivatives. The enzyme is highly conserved and is found in a wide variety of prokaryotes and eukaryotes. During fermentation by Rhizopus oryzae, pyruvate is primarily converted to lactic acid, but other by-products such as fumaric acid, ethanol and glycerol are also produced. In this fungus, there is evidence that fumaric acid production is predominantly from cytosolic oxaloacetate that is converted from pyruvate by pyruvate carboxylase (Osmani, S. A., et al., Eur. J. Biochem. 147:119-128 (1985)).
Active pyruvate carboxylase consists of four identical subunits arranged in a tetrahedron-like structure. Each subunit contains three functional domains: the biotin carboxylation domain, the transcarboxylation domain and the biotin carboxyl carrier domain (Jitrapakdee, S., et al., Biochem. J. 340:1-16 (1999)). Pyruvate carboxylases contain the prosthetic group biotin, which is covalently bound to the amino group of a specific lysine residue. The overall reaction catalyzed by pyruvate carboxylase involves two partial reactions that occur at spatially separate subsites within the active site, with the covalently bound biotin acting as a mobile carboxyl group carrier. In the first partial reaction, biotin is carboxylated using ATP and HCO3− as substrates, while in the second partial reaction, the carboxyl group from carboxybiotin is transferred to pyruvate (Attwood, P. V., Int. J. Biochem. Cell Biol. 27:231-249 (1995)).

Pyruvate carboxylase was first described by (Utter, M. F., et al., J. Biol. Chem. 235:17-18 (1960)) in the course of defining the gluconeogenic pathway in chicken liver. Native pyruvate carboxylase from a number of sources, including bacteria, yeast, insects and mammals, consists of four identical subunits of approximately 120-130 kDa. Pyruvate carboxylases from many sources possess a reactive lysine residue that is essential for full enzymatic activity. Sequencing of cDNA encoding pyruvate carboxylase, as well as limited proteolysis and primary structure comparisons, have shown that pyruvate carboxylases from different species contain ATP, pyruvate, and biotin binding domains (Jitrapakdee and Wallace (1999); Koffas, M. A., et al., Appl. Microbiol. Biotechnol. 50:346-352 (1998)). In S. cerevisiae there are two pyruvate carboxylase isoenzymes (PYC1 and PYC2) encoded by separate genes (Stucka, R., et al., Mol. Gen. Genet. 229:307-315 (1991); Walker, M. E., et al., Biochem. Biophys. Res. Commun. 176:1210-1217 (1991)) while in mammals, no tissue-specific isoenzymes have been reported. Pyruvate carboxylase is most effectively activated by long-chain acyl-CoA derivatives, such as palmitoyl-CoA, and is inhibited by aspartate and 2-oxoglutarate (Osmani, S. A., et al., Ann. N.Y. Acad. Sci. 447:56-71 (1985)).
Fermentations with the fungus Rhizopus are often advantageous because the organism is able to produce optically metabolites, such as pure L-(+)-lactic acid. Therefore, the quality of the final product is considered to be superior to that obtained by bacterial fermentations. Furthermore, L-(+)-lactic acid is more desirable for making poly-lactic acid. (U.S. Pat. No. 6,268,189). Additionally, Rhizopus can grow in chemically simple medium without the need for complex components such as yeast extracts (Skory, C. D. Curr. Microbiol. 47:59-64 (2003)). Nevertheless, the efficiency of lactic acid and fumaric acid production (the amount of available carbon diverted to lactate or fumarate production) in Rhizopus generally is low as compared to bacterial fermentations. There also is little known in the art about gene regulatory elements for Rhizopus. There is a need for a method of increasing the efficiency and amount of lactate and fumarate production in Rhizopus. 